1. Technical Field
This invention relates generally to radio frequency (RF) transmitters and, more particularly, to a balanced oscillator and transmitter circuit having two oscillators connected in a back-to-back configuration and sharing a common tank circuit for radiating RF signals with enhanced power output.
2. Discussion
Compact radio frequency (RF) transmitters are widely employed to transmit radiating signals for use in connection with remote signal communication systems. For instance, a wide variety of compact transmitters are commonly used with automotive vehicle remote keyless entry systems for remotely controlling access to a vehicle in addition to controlling other vehicular functions such as alarm system features, trunk release, etc. Similarly, compact transmitters are also widely used for remotely controlling the operations provided with automatic garage door systems, electronic sound systems, televisions and VCRs. The types of compact hand held transmitters often employed for such purposes are usually battery operated and are generally required to accommodate a compact enclosure and to exhibit efficient power usage. According to well known signal communication systems, the transmitter typically radiates an encoded RF signal with a predetermined carrier frequency. Generally speaking, the radiating signal is picked up with a remote receiver and processed, if necessary, and then provided as a control signal to a motor controller or other control device for a given system.
A number of conventional RF transmitters currently employ a single Colpitts oscillator for providing a local oscillation signal in the transmitter circuit. As an example, FIG. 1 illustrates a conventional transmitter circuit 10 employing a conventional single Colpitts-type oscillator which produces the local oscillation signal for transmission from an antenna element. This particular conventional transmitter circuit 10 has been commonly used in connection with a remote controlled keyless entry system for automotive vehicles.
More particularly, the conventional transmitter circuit 10 of FIG. 1 has a single Colpitts oscillator made up of a Colpitts-configured transistor Q1, and an input tank circuit. The input tank circuit typically includes a series resonator such as a surface acoustic wave (SAW) resonator 12 as well as feedback capacitors C1 and C2. The oscillator also includes a number of biasing resistors. The conventional transmitter circuit 10 also has an inductor L1 that is employed as a radiating antenna element for radiating the RF output signal. The Colpitts-configured transistor Q1 of the conventional transmitter circuit 10 has a base terminal 14 coupled to the surface acoustic wave resonator 12 and a collector terminal 16 coupled to the inductor L1. Transistor Q1 also has an emitter terminal 18 coupled to ground via a resistor R3. In addition, a feedback capacitor C2 is also coupled between emitter terminal 18 and ground, and therefore is connected in parallel with resistor R3. The feedback capacitor C1 is coupled between the collector terminal 16 and emitter terminal 18 of transistor Q1. Additionally, a third capacitor C3 is shown coupled between inductor L1 and ground for providing a large capacitance which maintains a constant DC voltage.
The circuit configuration according to the above-described conventional single oscillator transmitter circuit 10 typically receives a direct current (DC) voltage input signal V.sub.IN such as +6 volts DC. In addition, transmitter circuit 10 also receives a data input signal V.sub.data for encoding the RF carrier signal with pulse-width modulations. In response, circuit 10 operates to generate a radiating output signal via inductor L1. In doing so, the Colpitts-configured transistor Q1 acts as an amplification stage and, together with the tank circuit, generates a resonating signal which is provided as an oscillating current signal I through inductor L1. The transmission of current I through inductor L1 in turn causes transmission of the radiating output signal which is generally distributed as an electromagnetic field.
The above-described conventional single Colpitts oscillator approach has been suitable for providing RF signal transmission for applications such as the remote keyless entry system for a vehicle. However, the conventional single Colpitts oscillator is limited in the amount of power output that is generally achievable according to the conventional circuit configuration. Generally speaking, the size of the radiating inductor L1, may not simply be enhanced to achieve an increase in power output due to inherent limitations associated with the commonly employed circuit components. While attempts to enhance output power have considered optimization of component values, limitations remain which may include matching losses that are generally associated therewith. Further, rail-to-rail voltage swing limitations of the transistor Q1 tend to limit the amount of current flow through the conventional circuit. This in turn curtails the available power output that may be realized by a given transmitter circuit.
It is therefore one object of the present invention to provide for a remote balanced oscillator and transmitter for radiating RF signals and which is capable of achieving an enhanced power output over that of a conventional single oscillator type transmitter.
More particularly, it is an object of the present invention to provide for a balanced oscillator and transmitter circuit which employs a pair of balanced back-to-back Colpitts-type oscillators, a common tank circuit and a pair of series connected output tanks with antenna elements to achieve enhanced power output.
It is another object of the present invention to provide for a method of efficiently generating and transmitting an RF signal which may realize increased power output.
It is also a further object of the present invention to provide for a compact balanced oscillator and transmitter circuit and method of achieving an efficient transmission of a radiating signal which is capable of offering increased power output and is suitable for use in connection with a vehicular keyless entry system.